make ios build more like android build

The iOS build (especially Boost) is quite a bit different than it is for Android.  I've refactored the build so that iOS and Android are more similar.  In this pull request:
  * Android and iOS configure their Boost packages the same way and for the same packages
  * Android and iOS build ICU with the same flags
  * iOS builds Boost in the platform loop (like how Android does) rather than all in one go.  This greatly simplifies the build script, as all the lipo-ing can happen in the build-all script.
  * iOS Boost build now untars to the same place that Android does (which is just cleaner)
  * iOS Boost now produces the same .a files as Android (libboost_xxx as well as the combined libboost.a)